System and method for removing excess nitrogen from gas subcooled expander operations

ABSTRACT

A system and method for removing nitrogen from an intermediate stream in a gas subcooled process operation that processes natural gas into a sales gas stream and a natural gas liquids stream. The system and method of the invention are particularly suitable for use with gas subcooled process operations where the sales gas stream exceeds pipeline nitrogen specifications by up to about 3%, such as for reducing the nitrogen content of sales gas streams to levels permissible for pipeline transport.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system and method for separating nitrogenfrom methane and other components of natural gas streams being processedinto a sales gas stream and a natural gas liquids (NGL) stream by theknown gas subcooled/expander process (GSP/expander process or simplyGSP). The system and method of the invention are particularly suitablefor use in connection with natural gas streams where the processed salesgas stream contains nitrogen in an amount that exceeds pipelinespecifications.

2. Description of Related Art

Nitrogen contamination is a frequently encountered problem in theproduction of natural gas. Transporting pipelines typically do notaccept natural gas containing more than about 4 mole percent inerts,such as nitrogen. Estimates indicate that as much as 25% of natural gasin the United States exceeds a typical 4% pipeline specification. Tocorrect this problem, the sales gas may be mixed or diluted with othergas to achieve the desired nitrogen specification. Alternatively, knownmethods of nitrogen removal such as a nitrogen rejection unit or NRUcomprised of two cryogenic fractionating columns, as described in U.S.Pat. Nos. 4,451,275 and 4,609,390 or comprised of a single fractionatingcolumn, as described in U.S. Pat. Nos. 5,141,544, 5,257,505, and5,375,422 may be used. However, dilution and full-blown NRU installationand operation are expensive for the gas processor. Additionally, acomplete stand-alone NRU, which is capable of removing large percentagesof nitrogen, may not be necessary or economically feasible for a gassubcooled process where the sales gas exceeds the nitrogen specificationby only a small amount.

SUMMARY OF THE INVENTION

The system and method disclosed herein facilitate the economicallyefficient removal of nitrogen from methane by modifying a conventionalGSP system and its method of operation for nitrogen removal. The systemand method of the invention are particularly suitable for use where thefeed gas to a conventional GSP and expander system contains sufficientnitrogen and other inerts that the resultant sales gas contains a higherpercentage of nitrogen (or inerts) than is permitted by the operatingspecifications of a particular pipeline. For example, conventional GSPprocessing of raw natural gas normally containing 3-10% nitrogen mayproduce a sales gas that exceeds the nitrogen specification set by thetransporting pipeline by up to 3% (that is, sales gas nitrogen contentsof about 6-7%, with a typical pipeline specification being around 3-4%nitrogen). Through use of the present invention, the nitrogen content ofthe sales gas can be reduced to levels that are acceptable for pipelinetransport at a capital cost and with horsepower requirements that aresignificantly lower than those required by use of a conventionalstand-alone NRU unit.

According to one embodiment of the invention, a system and method aredisclosed for strategically integrating an NRU into a typicalGSP/expander operation. According to known GSP methods, a subcoolerreduces the temperature of a process stream prior to feeding the top ofa demethanizer column. Through use of this embodiment of the invention,a portion of the process stream that normally feeds the top of thedemethanizer column after passing through the subcooler serves as theNRU feed gas stream. The NRU feed stream passes through a singlefractionating column and other processing equipment to produce a treatedgas stream with reduced nitrogen content. That treated gas stream isthen reintroduced into the typical GSP operation as a portion of thefeed to the top of the demethanizer column for further processing. Theresult is a processed sales gas (or residue gas) stream having anitrogen content within typical pipeline specifications withoutadversely impacting the production of NGL product.

There are several advantages to the system and method disclosed hereinnot previously achievable by those of ordinary skill in the art usingexisting technologies. These advantages include, for example, an abilityto produce sales gas meeting typical pipeline specifications fornitrogen content without diluting the sales gas prior to transport andwithout requiring any additional dehydration or carbon dioxide removalprior to processing the gas for removal of nitrogen. Although thepresent system and method has the disadvantage of higher capital costsassociated with additional equipment for the NRU and higher operatingcosts for that equipment, compared to a GSP operation without an NRU,the costs of such are sufficiently offset by the savings in having salesgas within pipeline specification and savings in operating costsachieved by strategically placing the NRU within the GSP operation totake advantage of inter-operational efficiencies, such as heat exchangebetween process stream. Additionally, the costs of the NRU according tothe present system are reduced as compared to either a two-column orsingle column NRU operated externally to the GSP system. Such known NRUsystems have higher capital and operating costs associated with variouspieces of equipment typically used in such systems, such as theadditional fractionating column (in the two-column system), equipment toremove water and carbon dioxide, and multiple heat exchangers. Thecapital costs and operating expenditures for implementing the system andmethod of the present invention are believed to be around 25-50% of thecosts of conventional full-blown, stand-alone NRU systems. Additionally,the NRU system and method of the present invention do not substantiallyinterfere with NGL production and may enhance NGL recovery over a GSPsystem without the NRU system and method of the present invention.

Those of ordinary skill in the art will appreciate upon reading thisdisclosure that references to separation of nitrogen and methane usedherein refer to processing NRU feed gas to produce variousmulti-component product streams containing large amounts of theparticular desired component, but not pure streams of any particularcomponent. One of those product streams is a rejected nitrogen stream,which is primarily comprised of nitrogen but may have small amounts ofother components, such as methane and ethane. Another product stream isan intermediate stream, which is primarily comprised of methane but mayhave small amounts of other components, such as nitrogen, ethane, andpropane, that feeds the GSP demethanizer column to produce a sales gasstream within pipeline specifications.

Those of ordinary skill in the art will also appreciate upon readingthis disclosure that additional processing sections for removing carbondioxide, water vapor, and possibly other components or contaminants thatare present in the GPS feed stream or NRU feed stream, can also beincluded in the system and method of the invention, depending uponfactors such as, for example, the origin and intended disposition of theproduct streams and the amounts of such other gases, impurities orcontaminants as are present in the GSP feed stream or NRU feed stream.However, additional removal of carbon dioxide and water vapor from theNRU feed stream are not needed to achieve a sales gas stream withinpipeline specifications and the system and method of the invention willnot be adversely impacted by the presence of small amounts of suchcontaminants in the NRU feed stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The system and method of the invention are further described andexplained in relation to the following drawings wherein:

FIG. 1 is a simplified process flow diagram illustrating principalprocessing stages of a typical prior art GSP/expander operation;

FIG. 2 is a simplified process flow diagram illustrating principalprocessing stages of an embodiment of a system and method for separatingnitrogen from process streams in a GSP/expander operation;

FIG. 3 is a simplified process flow diagram illustrating principalprocessing stages of the NRU portion of an embodiment of a system andmethod for separating nitrogen from process streams in a GSP/expanderoperation;

FIG. 4 is a more detailed process flow diagram illustrating theprocessing stages of the simplified process flow diagram of FIG. 2;

FIG. 5 is a more detailed process flow diagram illustrating the NRUprocessing stage of the simplified process flow diagram of FIG. 3;

FIG. 6 is a simplified process flow diagram illustrating principalprocessing stages for another embodiment of the system and method forseparating nitrogen from process streams in a GSP/expander operation.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 (prior art) depicts the basic processing stages of a knownGSP/expander system 10. GSP feed gas 12 is fed through GSP primary heatexchange and primary separation block 14 and exits as streams 16 and 36.GSP primary heat exchange and primary separation block 14 contains oneor more separators, one or more heat exchangers, a compressor forcompressing the sales gas 50 and other equipment (such as valves,splitters, and mixers), which are known to those of ordinary skill inthe art. Stream 36 feeds a demethanizer column 40 and stream 16 passesthrough a GSP subcooler 26, exiting as stream 32 which also feedsdemethanizer column 40. Stream 22, split off from stream 16 passesthrough expander 24, exiting as stream 28 which also feed demethanizercolumn 40. The demethanizer column 40 produces an NGL product stream 42and an overhead stream 44. Overhead stream 44 passes through GSPsubcooler 26, exiting as stream 46, which then passes through the heatexchangers and compressor in block 14, exiting as sales gas stream 50,containing primarily methane. Depending on the composition of the GSPfeed stream 12, the resulting sales gas stream 50 may contain too muchnitrogen to meet pipeline specifications. For example, a GSP feed stream12 containing 4% nitrogen would result in a sales gas stream 50 with anitrogen content in excess of 4%, requiring further processing ordilution to meet pipeline specifications.

FIG. 2 depicts the basic processing stages of the system and methodaccording to a preferred embodiment of the invention. The system 100comprises processing equipment typically found in GSP operations, with afew modifications to permit insertion of NRU system 300 in the processas described more fully below. System 100 of the invention includesprocessing block 114, which contains the GSP primary heat exchanger(s)(one or more) and primary separators(s) (one or more) and a compressor,as well as other equipment (such as valves, splitters, and mixers),known to be used in a typical GSP operation. GSP feed stream 112 passesthrough GSP primary processing block 114, exiting as streams 116, 118,222, 136, and 138. Streams 116 and 118 pass through GSP subcooler 126,exiting as cooled streams 122 and 124. Stream 122 is the feed stream forNRU system 300. Two streams, treated gas stream 128 and refrigerantrecycle stream 130, are returned to system 100 after processing in NRUsystem 300, as depicted in FIG. 3. Cooled stream 124 is mixed withtreated gas stream 128 to form stream 132, which feeds a demethanizercolumn 140. Stream 228 mixes with refrigerant recycle stream 130 to formstream 134, which also feeds demethanizer column 140. Streams 136 and138 also feed demethanizer column 140. The demethanizer column 140produces an NGL product stream 142 and an overhead stream 144. Theoverhead stream 144 passes through GSP subcooler 126, exiting as stream146, which then passes through the heat exchanger(s) and compressor inblock 114, exiting as sales gas stream 150, containing primarilymethane. For a GSP feed stream containing around 4% nitrogen, the salesgas stream 150 will only contain around 3% nitrogen, which is withintypical pipeline specifications.

FIG. 3 depicts the basic processing stages of the NRU portion of thesystem and method according to a preferred embodiment of the invention.The NRU system 300 comprises a heat exchanger 302, a nitrogen rejectioncolumn 310, and a refrigerant recycle block 356, as well as otherequipment (such as valves, separators, and mixers, which are notdepicted in FIG. 3). Stream 122 exiting GSP subcooler 126, (depicted inFIG. 2) feeds the NRU heat exchanger 302 and exits as cooled stream 308,which feeds nitrogen rejection column 310. Nitrogen rejection column 310produces overhead stream 318 and bottom stream 322. Overhead stream 318passes through heat exchanger 302, exiting as warmed nitrogen ventstream 320. Bottom stream 322 is split into streams 354 and 340, both ofwhich pass through heat exchanger 302. Stream 354 is warmed, exiting astreated gas stream 128, which is returned to system 100 for furtherprocessing. Stream 340 is also warmed, exiting as stream 342, whichpasses through refrigerant recycle block 356 before returning to heatexchanger 302 as stream 350 and exits as refrigerant recycle stream 130.The cooled refrigerant recycle stream is returned to system 100 forfurther processing.

Systems 100 and 300 are depicted in greater detail in FIGS. 4 and 5.Referring to FIG. 4, a 100 MMSCFD GSP feed stream 112 containingapproximately 4% nitrogen and 70% methane at 120° F. and 750 psig issplit into streams 152 (67.5%) and 158 (32.5%) by splitter 151. Stream152 passes through heat exchanger 154 from which it emerges as stream156, having been cooled to 72° F. This cooling is the result of heatexchange with another process stream, 230, discussed later. Stream 158passes through heat exchanger 160 and exits as stream 162, having beencooled to 30.6° F. Streams 156 and 162 are mixed together by mixer 164to form stream 166 at 57.3° F. and 743 psig.

Stream 166 feeds a first separator 168 to produce a first overhead vaporstream 174 and a first bottom liquid stream 170. Bottom stream 170 has aflow rate of approximately 1.9 MMSCFD at 738 psig and 57° F., whichdrops to 265 psig and 31.2° F. after exiting Joule-Thomson (JT) valve172 as stream 138. Stream 138, containing 0.52% nitrogen, feeds a lowerstage of demethanizer column 140. JT valve 172 is capable of cooling bythe well-known Joule-Thomson effect, but in post-start up, steady stateoperation the valve provides less actual thermal cooling, but doesprovide the necessary pressure reduction for stream 138 prior to feedingdemethanizer column 140. Overhead stream 174 has a flow rate ofapproximately 98 MMSCFD at 738 psig and 57° F. before passing throughheat exchanger 176 to exit as stream 178 at −40° F. This cooling is theresult of heat exchange with process streams 146 (discussed below) andstream 258, which originates from a second process feed stream 240.Stream 240 contains 97% propane at 36° F. and 60 psig and passes throughJT valve 242, exiting as stream 244 having been cooled by expansion to−35° F. and a pressure of 3.6 psig. Stream 244 feeds a vertical standpipe 246, where the vapor rises and exits as stream 248 and the liquidexits the bottom of stand pipe 246 as stream 254. Stream 254 has avolumetric flow rate of 276.7 sgpm as it passes through a length ofvertical pipe 256 exiting as stream 258, with a slight increase inpressure. Stream 258 passes through heat exchanger 176 and exits asstream 260, having been warmed to 10° F. Stream 260 mixes with vaporstream 248 in mixer 250 forming stream 252 at −9.5° F. and 3.6 psig.

After exiting heat exchanger 176, stream 178 feeds a second separator180 to produce a second overhead vapor stream 196 and a second bottomliquid stream 182. Second bottom stream 182, with a flow rate ofapproximately 31 MMSCFD at 733 psig and −40° F., is split into stream136 (99.99%) and 186 (0.01%) by splitter 184. Stream 136, containing 1%nitrogen, feeds demethanizer column 140. Stream 186 passes through GSPsubcooler 126, exiting as stream 188. Stream 188, at 728 psig and −97.7°F., passes through a second JT valve 190, exiting as stream 124, at 215psig and −121° F. Stream 124 mixes with treated gas stream 128 (from NRUsystem 300) in mixer 192 to form stream 132. Stream 132, at a flow rateof 26.6 MMSCFD and containing around 0.5% nitrogen, feeds the top ofdemethanizer column 140 at 210 psig and −162.2° F.

Second overhead stream 196 exits second separator 180 with a flow rateof approximately 67.2 MMSCFD at 733 psig and −40° F. Stream 196 is splitinto stream 202 (45%) and stream 116 (55%) by splitter 198. Stream 116passes through GSP subcooler 126, exiting as NRU feed stream 122. Stream202 may be split into stream 206 and stream 218 by splitter 204;however, in this example of a preferred embodiment of the system andmethod of according to the invention, the entirety of stream 202 isdirected to stream 218. Valve 220 controls stream 218, but stream 220exits valve 220 as stream 222 at substantially the same temperature andpressure as stream 218. Stream 222 passes through expander 224 and exitsas stream 226, with the pressure having dropped from 730 psig to 225psig. If stream 206 is used, it passes through a third JT valve 208,exiting as stream 210. Stream 210 would then be mixed with stream 226 inmixer 212 to form stream 214. Stream 214 flows through a length of pipe(depicted as 216), over which there is a slight pressure drop, becomingstream 228. Stream 228, at a flow rate of 30.2 MMSCFD and containingaround 5.5% nitrogen, mixes with refrigerant recycle stream 130 (fromNRU system 300), at a flow rate of 8.2 MMSCFD and containing around 0.5%nitrogen, in mixer 194 to form stream 134. Stream 134, at a flow rate of38.4 MMSCFD and containing around 4.4% nitrogen, feeds demethanizercolumn 140.

Demethanizer column 140 separates feed streams 132, 134, 136, and 138into overhead stream 144 and bottoms stream 264. Stream 264 from thebottom of the demethanizer column 140 is directed to reboiler 266 thatreceives heat (designated as energy stream Q-110) from heat exchanger160. Stream 264 is at approximately 15.4° F. and 206 psig and contains anegligible amount of nitrogen, 2.6% methane, 58.3% ethane, and 29.6%propane. The demethanizer column 140 also receives heat from heatexchanger 176, designated by energy stream Q-114. Liquid stream 270exits reboiler 266 and feeds separator 272 where it is separated into anNGL stream 274 and a vapor stream 278. Stream 274 passes through pump276, exiting as an NGL product stream 142 at approximately 49° F. and1200 psig and 470 sgpm. NGL product stream 142 contains a negligibleamount of nitrogen, 52.8% ethane, 34% propane, and 1% methane, and issuitable for sale or further processing. Pump 276 requires an energyinput, designated as Q-118. Vapor stream 278 at 0.035 MMSCFD, 34.2° F.,205 psig and containing 7.9% methane is recycled to the bottom ofdemethanizer column 140. Vapor stream 268, containing 7.9% methane,exits reboiler 266 at 34.4° F., 206 psig and is also recycled to thebottom of demethanizer column 140.

Overhead stream 144 exits demethanizer column 140 at −147.5° F. and 200psig, with a volumetric flow rate of 73.4 MMSCFD, and containingapproximately 2.9% nitrogen, 94.5% methane, and 2.5% ethane. Stream 144passes through subcooler 126, exiting as stream 146, having been warmedto −50° F. Stream 146 then passes through heat exchanger 176, exiting asstream 230, having been warmed to 44.5° F. Stream 230 then passesthrough heat exchanger 154, exiting as stream 232, having been warmed to109.7° F. Stream 232 passes through a JT valve 234, exiting as stream236 having a slight drop in pressure. Stream 236 passes throughcompressor 238 (powered by energy from expander 224, designated byenergy stream Q-116) exiting as sales gas stream 150. Sales gas streamis at 133° F. and 207.7 psig and contains 2.9% nitrogen, making itsuitable for sale within typical pipeline specifications. In a typicalGSP operation, a feed stream nitrogen content of around 4% would resultin a sales gas nitrogen content greater than 4%, falling outside mostpipeline specifications. In this preferred embodiment of the invention,the sales gas stream 150 nitrogen content is reduced from the feedstream nitrogen content by slightly more than 1%. This reduction innitrogen is possible by the addition of the NRU system 300, depicted inFIG. 5.

Referring to FIG. 5, NRU feed stream 122, containing 5.5% nitrogen andhaving a temperature of −97.7° F. and a pressure of 728 psig enters NRUsystem 300. According to this embodiment of the present invention, it isnot necessary to remove water vapor or carbon dioxide from the NRU feedstream 122, although additional steps and equipment may be added to doso if desired. Methods for removing water vapor, carbon dioxide, andother contaminants are generally known to those of ordinary skill in theart and are not described herein. NRU feed stream 122 passes throughheat exchanger 302 exiting as stream 304, having been cooled to −185° F.Stream 304 passes through JT valve 306, reducing the pressure of exitingstream 308 to 250 psig. Stream 304 feeds nitrogen rejection column 310.Stream 322 exits the bottom of nitrogen rejection column 310, containing1% nitrogen, and feeds virtual reboiler 324. Although depicted in FIG. 5as a separate piece of equipment, preferably virtual reboiler 324 ispart of other process equipment as will be understood by those ofordinary skill in the art. Vapor stream 326, containing 2.8% nitrogenand 96.6% methane, exits virtual reboiler 324 and is fed back into thebottom of nitrogen rejection column 310. Liquid stream 328, containing0.5% nitrogen, 87% methane and 9.9% ethane exits virtual reboiler 234 at−159° F. and 253 psig. Stream 328 is split into stream 332 (23.5%) andstream 334 (76.5%) by splitter 330.

Stream 332 passes through JT valve 335 and exits as stream 336 havingbeen cooled by expansion to −240° F., with a pressure of 12 psig. Stream336 passes through virtual condenser 338 and exits as stream 340, warmedto −210° F. Stream 340 passes through heat exchanger 302 and exits asstream 342 warmed to 90° F. Stream 342 feeds a refrigerant recyclesystem 356, first passing through compressor 344 (supplied by energystream Q-318) and exiting as stream 346 at a temperature of 573° F. anda pressure of 225 psig. Stream 346 passes through cooler 348 (releasingenergy stream Q-316) and is cooled to 120° F. as exiting stream 350.Stream 350 passes through heat exchanger 302, exiting as refrigerantrecycle stream 130 at −90° F. and 215 psig. Refrigerant recycle stream130 is returned to system 100 for further processing as described above.

Stream 334 passes through JT valve 352, with a pressure drop of 38 psigand a decrease in temperature of around 7° F. as it exits as stream 354.Stream 354 passes through heat exchanger 302, exiting as treated gasstream 128. Treated gas stream 128 contains only 0.5% nitrogen, comparedto the 5.5% nitrogen in NRU feed stream 122. Treated gas stream 128 isreturned to system 100 for further processing as discussed above.

Vapor stream 312 exits the top of nitrogen rejection column 310, havinga temperature of −203° F. and a pressure of 250 psig and containing57.5% nitrogen. Stream 312 feeds internal condenser 314 (which isdepicted as exterior to NRU column 310). Heat released from internalcondenser 314 (designated as energy stream Q-314) supplies virtualcondenser 338. Liquid stream 316 exits internal condenser 314 and isfeed back into NRU rejection column 310. Vapor stream 318, at −232° F.,passes through heat exchanger 302, exiting as rejected nitrogen stream320, warmed to 90° F. Rejected nitrogen stream 320 contains 85%nitrogen, 15% methane, and negligible amounts of ethane and propane. Theheat released from heat exchanger 302 supplies heat to nitrogenrejection column 310 (designated as energy stream Q-312) and virtualreboiler 324 (designated as energy stream Q-310).

EXAMPLE

The flow rates, temperatures and pressures of various flow streamsreferred to in connection with the discussion of the system and methodof the invention in relation to FIGS. 4 and 5, for a GSP feed streamflow rate of 100 MMSCFD and containing 4% nitrogen, 70% methane, 14.7%ethane, and 8.4% propane appear in Table 1 below. The values for theenergy streams referred to in connection with the discussions of thesystem and method of the invention in relation to FIGS. 4 and 5 appearin Table 2 below. The values discussed herein and in the tables beloware approximate values.

TABLE 1 FLOW STREAM PROPERTIES Stream Temper- Reference % % % % FlowRate ature Pressure Numeral N₂ CH₄ C₂H₆ C₃H₈ (lbmol/h) (deg. F.) (psig)112 4 69.9 14.8 8.4 10979.8 120 750.3 116 5.5 82.8 9.3 2.2 4057.2 −40733.3 122 5.5 82.8 9.3 2.2 4057.2 −97.7 728.3 124 1 44.8 46.4 20.6 0.34−121.1 215 128 0.5 87 9.9 2.3 2919.1 −162.2 210 130 0.5 87 9.9 2.3 899.6−90 215 132 0.5 87 9.9 2.4 2919.5 −162.2 210 134 4.4 83.7 9.4 2.2 4219.1−109.5 215 136 1 44.8 26.4 20.6 3395.2 −40 733.3 138 0.5 22.4 18.8 26.7207.6 31.2 265 142 neg. 1 52.8 34.1 2686 49.1 1200 144 2.9 94.5 2.5 0.048055.4 −147.5 200 146 2.9 94.5 2.5 0.04 8055.4 −50 195 150 2.9 94.5 2.50.04 8055.4 133.4 207.8 152 4 69.9 14.8 8.4 7412.6 119.9 748.3 156 469.9 14.8 8.4 7412.6 72 743.3 158 4 69.9 14.8 8.4 3567.3 119.9 748.3 1624 69.9 14.8 8.4 3567.3 30.6 743.3 166 4 69.9 14.8 8.4 10979.8 57.3 743.3170 0.5 22.4 18.8 26.7 207.61 57 738.3 174 4.1 70.8 14.7 8 10772.2 57738.3 178 4.1 70.8 14.7 8 10772.2 −40 733.3 182 1 44.8 26.4 20.6 3395.56−40 733.3 188 1 44.8 26.4 20.6 0.34 −97.7 728.3 186 1 44.8 26.4 20.60.34 −40 733.3 196 5.5 82.8 9.3 2.2 7376.6 −40 733.3 202 5.5 82.8 9.32.2 3319.5 −40 733.3 214 5.5 82.8 9.3 2.2 3319.5 −113.1 225 218 5.5 82.89.3 2.2 3319.5 −40 733.3 222 5.5 82.8 9.3 2.2 3319.5 −40.2 730.3 226 5.582.8 9.3 2.2 3319.5 −113.1 225 228 5.5 82.8 9.3 2.2 3319.5 −113.7 221.7230 2.9 94.5 2.5 0.04 8055.4 44.5 190 232 2.9 94.5 2.5 0.04 8055.4 109.7185 236 2.9 94.5 2.5 0.04 8055.4 109.4 180 240 0 0 1.5 97 2861.9 36.360.3 244 0 0 1.5 97 2861.9 −35 3.6 248 0 0 2.8 96.6 1276.4 −35 3.6 252 00 1.5 97 2861.9 −9.5 3.6 254 0 0 0.5 97.3 1585.5 −35 3.6 258 0 0 0.597.3 1585.5 −35 4.8 260 0 0 0.5 97.3 1585.5 10.2 3.8 264 neg. 2.6 58.329.6 3495.3 15.4 206 268 neg. 8 76.3 14.4 805.4 34.5 206 270 neg. 1 52.934.1 2689.9 34.5 206 274 neg. 1 52.8 34.1 2686 34.2 205 278 neg. 7.976.3 14.4 3.9 34.2 205 304 5.5 82.8 9.3 2.2 4057.2 −185 723.3 308 5.582.8 9.3 2.2 4057.2 −184.4 250.3 312 57.6 42.4 neg. neg. 968.3 −203249.8 316 48.6 51.4 neg. neg. 729.9 −232.2 249.8 318 85 15 neg. neg.238.4 −232.2 249.8 320 85 15 neg. neg. 238.4 90 244.8 322 1 89.3 7.7 1.84981.2 −162.2 252.8 326 2.8 96.6 0.6 0.02 1162.4 −159.1 252.8 328 0.5 879.9 2.3 3818.8 −159.1 252.8 332 0.5 87 9.9 2.3 899.6 −159.1 252.8 3340.5 87 9.9 2.3 2919.1 −159.1 252.8 336 0.5 87 9.9 2.3 899.6 −239.9 12340 0.5 87 9.9 2.3 899.6 −210 10 342 0.5 87 9.9 2.3 899.6 90 5 346 0.587 9.9 2.3 899.6 573.1 225 350 0.5 87 9.9 2.3 899.6 120 220 354 0.5 879.9 2.3 2919.1 −466 215

TABLE 2 ENERGY STREAM REPORT Energy Stream Energy Reference Rate PowerNumeral (Btu/h) (hp) From To Q-110 5.27E+06 2070.3 Heat ReboilerExchanger 266 160 Q-114   4E+06 1572.1 Heat Demeth. Exchanger Column 140176 Q-116 1.65E+06 649.2 Expander Compressor 224 238 Q-118 967034 380.1— Pump 276 Q-310 3.05E+06 1200 Heat Virtual Exchanger Reboiler 302 324Q-312 500000 196.5 Heat N₂ Reject. Exchanger Column 310 302 Q-314 1.8E+06 706.4 Internal Virtual Condenser Condenser 314 338 Q-3164.75E+06 1865.7 Cooler 348 — Q-318 4.88E+06 1918.7 — Compressor 344

Those of ordinary skill in the art will appreciate upon reading thisdisclosure that the values discussed above are based on the particularparameters and composition of the feed stream in the Example, and thatthe values can differ depending upon differences in operating conditionsand upon the parameters and composition of the GSP feed stream 112 andthe NRU feed stream 122.

FIG. 6 depicts the basic processing stages of another embodiment of theinvention, wherein the NRU processing stage 600 is located in analternate location compared to the NRU processing stage 300 as depictedin FIGS. 2 and 4. The system 400 comprises processing equipmenttypically found in GSP operations, with a few modifications to permitinsertion of system 600 in the process, as will be understood by thoseof ordinary skill in the art. System 400 of the invention includesprocessing block 414, which contains the GSP primary heat exchanger(s)(one or more) and primary separator(s) (one or more) and a compressor,as well as other equipment (such as valves, splitters, and mixers),known to be used in a typical GSP operation. GSP feed stream 412 passesthrough GSP primary processing block 414, exiting as streams 416 and436. A portion of stream 436 is split off into stream 486, with theremainder of stream 436 feeding demethanizer column 440. A portion ofstream 416 is split off into stream 522, which passes through expander524 exiting as stream 528. Stream 528 also feeds demethanizer column440. The remainder of stream 416 is mixed with stream 486 before passingthrough GSP subcooler 426, exiting as cooled stream 432, which alsofeeds demethanizer column 440. The demethanizer column 440 produces anNGL product stream 442 and an overhead stream 444. The overhead stream444 feeds NRU processing stage 600. Treated gas stream 428 and anitrogen reject stream 620 exit NRU processing stage 600. Treated gasstream 428 passes through subcooler 426, exiting as stream 446. Stream446 then passes through the heat exchanger(s) and compressor in block414, exiting as sales gas stream 450, containing primarily methane. Fora GSP feed stream containing around 4% nitrogen, the sales gas stream450 will only contain around 3% nitrogen, which is within typicalpipeline specifications. NRU processing stage 600 is similar to NRUprocessing stage 300 depicted in FIG. 5.

Those of ordinary skill in the art will appreciate upon reading thedisclosure in light of the accompanying drawings that the system andmethod of the present invention can be used with expander configurationsknown to be used in GSP operations other than those depicted in thedrawings and described herein. Other alterations and modifications ofthe invention will likewise become apparent to those of ordinary skillin the art upon reading this specification in view of the accompanyingdrawings, and it is intended that the scope of the invention disclosedherein be limited only by the broadest interpretation of the appendedclaims to which the inventor is legally entitled.

I claim:
 1. A method for removing excess nitrogen from a GSPfractionating column feed stream in a known gas subcooled processcomprising: at least one separation step wherein at least a portion of aGSP feed stream is separated into a first overhead stream and a firstbottoms stream and wherein at least a first portion of the first bottomsstream is a first feed stream for the GSP fractionating column, acooling step wherein a first portion of the first overhead stream iscooled to form a second feed stream for the GSP fractionating columnusing a subcooler for heat exchange with a second overhead stream, anexpanding step wherein a second portion of the first overhead stream isexpanded using an expander to form a third feed stream for the GSPfractionating column, and a fractionating step wherein the first, secondand third feed streams are separated into the second overhead stream anda second bottoms stream in the GSP fractionating column, the nitrogenremoval method comprising: separating a nitrogen removal feed streaminto a third overhead stream and a third bottoms stream in a singlesecond fractionating column; forming the nitrogen removal feed stream bycooling at least a portion of the second feed stream upstream of the GSPfractionating column by heat exchange with the third bottoms stream andthe third overhead stream, splitting the third bottoms stream into afirst portion of the third bottoms stream and a second portion of thethird bottoms stream; reintroducing at least the first portion of thethird bottoms stream after heat exchange back to the gas subcooledprocess as the second feed stream or portion of the second feed streamto the fractionating step of the gas subcooled process; and mixing thesecond portion of the third bottoms stream with the expanded portion ofthe first overhead stream to form the third feed stream.
 2. The methodof claim 1 further comprising: compressing and cooling the secondportion of the third bottoms stream after heat exchange with at least aportion of the second overhead stream; recycling the cooled secondportion of the third bottoms stream for additional heat exchange with atleast a portion of the second overhead stream and the third overheadstream; recycling the second portion of the third bottoms stream back tothe gas subcooled process; and wherein the third bottoms stream is splitinto the first portion of the third bottoms stream and the secondportion of the third bottoms stream prior to heat exchange with at leasta portion of the second overhead stream.
 3. A method for removing excessnitrogen from a GSP fractionating column feed stream in a known gassubcooled process comprising: at least one separation step wherein atleast a portion of a GSP feed stream is separated into a first overheadstream and a first bottoms stream and wherein at least a first portionof the first bottoms stream is a first feed stream for the GSPfractionating column, a cooling step wherein a first portion of thefirst overhead stream is cooled to form a second feed stream for the GSPfractionating column using a subcooler for heat exchange with a secondoverhead stream, an expanding step wherein a second portion of the firstoverhead stream is expanded using an expander to form a third feedstream for the GSP fractionating column, and a fractionating stepwherein the first, second and third feed streams are separated into thesecond overhead stream and a second bottoms stream in the GSPfractionating column, the nitrogen removal method comprising: separatinga nitrogen removal feed stream into a third overhead stream and a thirdbottoms stream in a single second fractionating column; forming thenitrogen removal feed stream by cooling at least a portion of the secondfeed stream upstream of the GSP fractionating column by heat exchangewith the third bottoms stream and the third overhead stream, splittingthe third bottoms stream into a first portion of the third bottomsstream and a second portion of the third bottoms stream; reintroducingat least the first portion of the third bottoms stream after heatexchange back to the gas subcooled process and mixing the first portionof the third bottoms stream with the cooled second portion of the firstbottoms stream to form the second feed stream or portion of the secondfeed stream to the fractionating step of the gas subcooled process. 4.The method of claim 1 wherein the third overhead stream comprises atleast about 80% nitrogen and the first portion of the third bottomsstream comprises less than about 2% nitrogen and at least about 80%methane.
 5. A system for removing excess nitrogen from a demethanizerfeed stream in a known gas subcooled process system comprising: ademethanizer, at least one separator upstream from the demethanizerwherein at least a portion of a GSP feed stream is separated into afirst overhead stream and a first bottoms stream and wherein at least afirst portion of the first bottoms stream is a first feed stream for thedemethanizer, a subcooler upstream from the demethanizer wherein a firstportion of the first overhead stream is cooled by heat exchange with asecond overhead stream; a first mixer to mix the cooled first portion ofthe first overhead stream with a first portion of a third bottoms streamto form a second feed stream for the demethanizer, an expander upstreamfrom the demethanizer wherein a second portion of the first overheadstream is expanded to form a third feed stream for the demethanizer, andwherein the first, second and third feed streams are separated into thesecond overhead stream and a second bottoms stream in the demethanizer;the nitrogen removal system comprising: a single fractionating columnwherein a nitrogen removal feed stream is separated into a thirdoverhead stream and a third bottoms stream; a splitter to split thethird bottoms stream into the first portion of the third bottoms streamand a second portion of the third bottoms stream; a heat exchanger forcooling at least a portion of the second feed stream upstream of thedemethanizer to form the nitrogen removal feed stream through heatexchange with the third bottoms stream and third overhead stream;wherein the third overhead stream comprises at least 80% nitrogen; andwherein the second portion of the third bottoms stream comprises lessthan 4% nitrogen and is reintroduced back to the gas subcooled processas a portion of the second feed stream.
 6. The system according to claim5 further comprising: a second mixer to mix the expanded second portionof the first overhead stream with the second portion of the thirdbottoms stream to form the third feed stream for the demethanizer. 7.The method of claim 3 further comprising: splitting the first bottomsstream into the first portion of the first bottoms stream and a secondportion of the first bottoms stream; cooling the second portion of thefirst bottoms stream in the subcooler through heat exchange with thesecond overhead stream; and mixing the second portion of the thirdbottoms stream with the expanded portion of the first overhead stream toform the third feed stream to the gas subcooled process fractionatingstep.